The present invention relates in general to motor control circuitry and pertains, more particularly, to a motor drive circuit for an AC motor such as a shaded pole motor. In accordance with the invention alternate control is provided wherein high speed operation the windings are directly driven from the AC source and for lower speed operation the drive circuit is adjustable to provide the capability of adjustable variable speed control. The control circuit of this invention may be used with, for example, a milk shake machine for mixing control.
One object of the present invention is to provide an improved drive control circuit for an AC motor. The motor of the present invention is preferably a shaded pole motor having advantages over a DC motor with regard to a cheaper cost and also with regard to the fact that the AC motor does not require mechanical brush commutation.
Another object of the present invention is to provide an improved AC motor drive circuit providing essentially two-speed control and wherein at least one of the speeds is preferably adjustable. In accordance with the present invention, high speed operation is provided by direct control of the windings of the motor from the AC source while the lower speed operation is via the adjustable control circuit.
A further object of the present invention is to provide an approved AC motor drive circuit that is relatively simple in construction having means for limiting energy consumption by at least partial recovery of energy used as a drive current to the windings.
To accomplish the foregoing and other objects of this invention, there is provided a motor drive circuit for driving the windings of an AC motor such as a shaded pole motor. The motor drive circuit comprises an oscillator or clock means and first and second semi-conductor switching means, each having at least an input control electrode and an output electrode. Means are provided, preferably including at least one bistable device coupled from the clock means for generating complementary cyclic signals for controlling the input control electrodes, respectively, of the semi-conductor switch means. The drive circuit also includes a load impedance means, which in a preferred embodiment comprises a parallel circuit of a capacitor and resistor coupled to the high voltage supply which may be a 150 volt supply. First and second unilateral conducting devices couple, respectively, from the output electrodes of the first and second semiconductor switching means to this load impedance means. Lastly, conductive connections are made from the semi-conductor switching means at their output electrodes to the motor windings.
In accordance with the present invention the control is essentially a two-speed control including a switch means for providing the two different modes of control. In the system of this invention means are provided for generating complementary drive signals and there is also provided an input means for receiving an AC drive voltage. The switch means may be operated in one mode for coupling the complementary drive signals to a series connection of the windings while in the other mode the switch means couples the AC drive voltage to a parallel connection of the windings. In the actual disclosed embodiment the motor windings comprise three windings of turns N, N/2 and N/2. In the one mode of operation, all three windings are connected in series by operation of the switch while in the other mode of operation, only the two N/2 windings are connected in parallel.